Hello,

I have a chance to buy enough surplus mineral wool batts (1.5 thick, 48”x16”) to make three layers of it above the ceiling in our shop – for a total of 4.5” of the stuff. If it is about R3 per inch (?), that is a bit more than R12 total. I would still need to add more insulation to get a better R-value above the shop. But from what I’ve read from other posts here, if I buy fiberglass batts I could just lay them down on top of the mineral wool. My interest in the mineral wool is to buy surplus (keep cost down), fire resistance, and I would have some left over to use in the house for sound attenuation in some of the interior walls and fire resistance around stairway framing (I have to buy all of it or none).
But… I’m new to all of this and I’m wondering if you all have any advice. For example, I haven’t got any quotes yet but I was thinking that a DIY installing batts is always going to be significantly less expensive than a blow-in application done by a pro, right? I didn’t have time to get quotes when I found this surplus material and it got me to thinking about insulation even though we’re not at that stage yet.

Thanks much!

one comment......... VERY ITCHY

is it more itchy than fiberglass batts?

Posted By Jelly on 13 Sep 2010 07:34 AM
is it more itchy than fiberglass batts?

In my opinion YES


Common Name(s): Mineral wool batts, mineral wool, rock wool, slag wool
R-value per Inch:  about 3.7
Strengths or Best Use: high recycled content, easy DIY installation, naturally fire resistant and can be used in close contact with stoves and chimneys, not damaged by moderate moisture contact
Weaknesses: not very air tight, subject to air movement around the batts, not widely available, fibers can cause indoor air quality problems (rare), itchy to install

Performance per pound is about the same as standard density fiberglass batts, but I'm not sure it buys you any fire-resistance. While not flammable itself, it's highly air-permeable- doesn't slow fire from crawling up stud bays without fireblocking.

In general, insulating a wall reduces it's fire-resistance from fire side to far side by not allowing the interior-side of the gypsum board to convection-cool into the cavity, causing it to fail sooner. More gypsum increases fire resistance. Higher density/lower air-permeabilty insulation increases the fire rating for vertical travel of the fire. Foam works pretty well, but has toxic smoke. Cellulose is ~15% fire-retardent by weight, and lower-air-perm, high density, and beats other fibers on fire resistance in some types of tests, but isn't appreciably better (or worse) in others.

DIY batts at full retail is typically about the same or slightly more expensive than letting the pros do it on any job of appreciable size. Blown insulation in attics is usually slightly cheaper (and better) than batts. (Open-blown cellulose in attics is a relatively easy DIY, but doing wall cavities takes more skill/knowledge.) Whether your discount on the surplus rock-wool is enough to beat blown goods at the same R isn't necessarily going to work, but it might. Beyond cost, blown goods tend to outperform batts, since there are inherently fewer voids/gaps/depressions than with batts. (But even a 3" overblow on top of batts can often fix most of the installation issues with them.)